The present invention relates to neural stimulators, e.g., cochlear implants, and more particularly to a technique for optimizing the number of channels a cochlear or other neural implant system should employ in order to enhance the performance of the implant system. In the case of a cochlear implant system, the present invention optimizes the number of channels the cochlear implant should use in order to enhance a user's ability to perceive and understand sound.
Electrical stimulation of predetermined locations within the cochlea of the human ear through an intra-cochlear electrode array is described, e.g., in U.S. Pat. No. 4,400,590. The electrode array shown in the '590 patent comprises a plurality of exposed electrode pairs spaced along and imbedded in a resilient curved base for implantation in accordance with a method of surgical implantation, e.g., as described in U.S. Pat. No. 3,751,615. The system described in the '590 patent receives audio signals, i.e., sound waves, at a signal processor (or speech processor) located outside the body of a hearing impaired patient. The speech processor converts the received audio signals into modulated RF data signals that are transmitted by a cable connection through the patient's skin to an implanted multi-channel intracochlear electrode array. The modulated RF signals are demodulated into analog signals and are applied to selected ones of the plurality of exposed electrode pairs in the intra-cochlear electrode so as to electrically stimulate predetermined locations of the auditory nerve within the cochlea.
U.S. Pat. No. 5,938,691, incorporated herein by reference, shows an improved multi-channel cochlear stimulation system employing an implanted cochlear stimulator (ICS) and an externally wearable speech processor (SP). The speech processor employs a headpiece that is placed adjacent to the ear of the patient, which receives audio signals and transmits the audio signals back to the speech processor. The speech processor receives and processes the audio signals and generates data indicative of the audio signals for transcutaneous transmission to the implantable cochlear stimulator. The implantable cochlear stimulator receives the transmission from the speech processor and applies stimulation signals to a plurality of cochlea stimulating channels, each having a pair of electrodes in an electrode array associated therewith. Each of the cochlea stimulating channels uses a capacitor to couple the electrodes of the electrode array.
Other improved features of a cochlear implant system are taught, e.g., in U.S. Pat. Nos. 5,626,629; 6,067,474; 6,157,861; 6,219,580; 6,249,704; and 6,289,247, each of which patents is incorporated herein by reference. Further enhancements are disclosed, e.g., in pending and co-owned U.S. patent application Ser. No. 10/218,645, filed Aug. 13, 2002, and U.S. patent application Ser. No. 10/218,616, filed Aug. 13, 2002, each of which patent applications is also incorporated herein by reference.
The implantable cochlear stimulators described in the '629, '474, '861 and '580 patents are also able to selectively control the pulse width of stimulating pulses that are applied through the electrode array to the cochlea, as well as the frequency at which the stimulating pulses are applied.
New generation cochlear implants have enhanced processing power, and can provide multiple platforms for delivering electrical stimuli to the auditory nerve. This includes the ability to deliver high frequency pulsitile stimulation made up of current pulses of controlled amplitude, width and frequency. Such new generation cochlear implants are frequently referred to as a “bionic ear” implant.
As the art of cochlear stimulation has advanced to produce bionic ear implants, the implanted portion of the cochlear stimulation system, and the externally wearable processor (or speech processor) have become increasingly complicated and sophisticated. Much of the circuitry previously employed in the externally wearable processor has been moved to the implanted portion, thereby reducing the amount of information that must be transmitted from the external wearable processor to the implanted portion.
As the complexity of the bionic ear implants has increased, the amount of control and discretion exercisable by an audiologist in selecting the modes and methods of operation of the cochlear stimulation system has also increased dramatically. For example, it is no longer possible to fully control and customize the operation of the cochlear stimulation system through the use of, for example, switches located on the speech processor. As a result, it has become necessary to utilize an implantable cochlear stimulator fitting system to establish the operating modes and methods of the cochlear stimulation system and then to download such programming into the speech processor. One such fitting system is described in the '629 patent. An improved fitting system is described in the '247 patent.
The '247 patent, in addition to showing an improved fitting system, also highlights representative stimulation strategies that may be employed by a multichannel stimulation system. Such strategies represent the manner or technique in which the stimulation current is applied to the electrodes of an electrode array used with the stimulation system. Such stimulation strategies, all of which apply current pulses to selected electrodes, may be broadly classified as: (1) sequential or non-simultaneous (where only one electrode receives a current pulse at the same time); (2) simultaneous (where the electrodes associated with more than one channel receive current stimuli at the same time); or (3) partially simultaneous pulsitile stimulation (where only a select grouping of the electrodes receive stimuli at the same time in accordance with a predefined pattern).
Recognition of speech sounds by cochlear implant recipients is based upon the ability of the sound processor to represent the time-varying acoustic patterns such that the listener can resolve the important temporal and spectral characteristics of those acoustic patterns. Differences in how speech sounds are made are reflected in the acoustic spectrum. These acoustic cues contain the manner in which a sound is made in the vocal tract, the place at which the vocal tract constricts the airstream in the oral cavity and whether or not the sound's production involves vocal fold vibration.
In cochlear implant processors, the above-described spectral cues are distributed across some number of channels. In modern cochlear processors the number of channels selected for a stimulation sequence can exceed 12-15, with various degrees of simultaneity in terms of data presentation. While this may allow for the allocation of spectral details to be transmitted to the auditory nerve along a large number of physical channels, problems such as spatial channel interaction and the inability to resolve place of stimulation among adjacent channels may actually lead to poorer discrimination. That is, systematic assessment of phoneme recognition by cochlear implant users has shown that while the features of manner of articulation and voicing are well transmitted, place cues are less well resolved. As a result, it has been determined that having a greater number of stimulation channels by itself is not always sufficient to provide improved understanding. To the contrary, in some instances, increasing the number of channels has actually caused a decrease in the user's understanding. Hence, it is seen that what is needed is a clinically relevant way of optimizing the number of channels that are used by the cochlear implant system so that the user's ability to improve understanding is maximized.
While assessing spatial (as well as temporal) channel interaction could be helpful in optimizing channels to be used in a stimulation sequence, the procedures necessary to carry out such assessment are far too cumbersome to be used clinically. An easier way to look at the issue of spectral confusion among adjacent stimulation channels is to determine the implant user's ability to differentiate channels based upon the perceived pitch derived from the place of stimulation. While clinical procedures do exist for “rating” (same/different task), “ranking” (high/low) or “scaling (numerical estimation) the pitch perceived by stimulating channels along an electrode array, such procedures do not provide a systematic method for quantifying the magnitude and direction of errors along the place-pitch continuum. Without such a method or technique, the reduction or re-ordering of stimulation channels may only be accomplished in a haphazard way, thereby penalizing patient performance. What is needed, therefore, is a method or technique for systematically identifying an optimum number and/or an optimum ordering of stimulation channels along the place-pitch continuum so that the patient's performance may be enhanced.